Development of an effective vaccine for prevention of HIV-1 infection is a global priority. While the RV144 trial using ALVAC prime, AIDSVAX B/E boost resulted in an estimated 31% vaccine efficacy, the protection induced was neither sufficiently robust for deployment, nor of sufficient durability for sustained vaccine efficacy.8 In the RV144 trial, antibodies capable of neutralizing transmitted/founder viruses were not induced; rather the hypothesis is that antibodies targeted to the Env V2 region protected by effector mechanisms other than virus neutralization, such as antibody-dependent cellular cytotoxicity (ADCC).9a, 10, 11 
By contrast, antibodies capable of broadly neutralizing HIV-1 strains (BnAbs) have been isolated from HIV-1 chronically-infected subjects.1 These antibodies are targeted to four general HIV-1 envelope targets: the gp41 membrane proximal external region (MPER), and on gp120, the CD4 binding site (CD4bs), the V1V2 glycan site, and several gp120 glycan sites.2 However, only approximately 20% of infected subjects produce BnAbs, and then, only after years of infection.12 When infused into rhesus macaques, BnAbs can protect against infection with chimeric simian-human immunodeficiency viruses (SHIVs).13 However, BnAbs have not been successfully induced by vaccine constructs thus far.
A recently described set of epitopes to which potent BnAbs do bind are defined by the PGT121, 125 and 128 mAbs isolated from HIV-1 chronically infected subjects.4,6 These antibodies are able to recognize a peptide-glycan epitope around the base of the gp120 V3 loop and includes N-linked glycans at amino acids 301 and 332.6 These antibodies are the most potent BnAbs isolated to date, and are able to protect rhesus macaques from SHIV challenge at extremely low plasma levels.14 Thus, induction of antibodies with specificities like these PGT antibodies is a key goal of HIV-1 vaccine development.
Many reasons can be envisioned to account for the difficulties encountered in inducing such BnAbs. The heavily glycosylated envelope could well obstruct antibody access to BnAb sites.2 The dominance of non-neutralizing epitope responses compared to the non-dominance of BnAb epitope responses might result in an inability of the B cell response to BnAb epitopes to either be induced or to be sustained.1 To date, all BnAbs that have been isolated from chronically infected subjects exhibit unusual traits including high levels of somatic mutations, long heavy chain third complementarity determining regions (HCDR3s), and polyreactivity for non-HIV-1 antigens—all traits of antibodies that are potentially susceptible to control by host tolerance mechanisms.1,7 
Thus, it seems likely from the unusual nature of BnAbs that unusual strategies will be required for their induction. Several discovery platforms have been proposed, including definition of BnAb epitope structures on the surface of native Env trimers,2 identification of Env constructs that bind mature BnAbs and their ancestor antibodies avidly,1b and immunization with sequential Env constructs isolated during the course of HIV-1 infection.15 However, none of these modes of operation take into account the heterogeneity of forms of recombinant Envs,16 and none prevent dominant, non-neutralizing antibody epitopes from inducing antibodies that out-compete BnAb-producing B cells targeted at sub-dominant Env BnAb epitopes. The ability to synthesize completely homogeneous glycopeptides that mimic the antigenic nature of native envelope proteins would facilitate the generation of constructs that can be configured to express primarily or only BnAb sub-dominant epitopes, thus limiting the likelihood of a dominant non-neutralizing response to be induced.
New chemical methods have been developed to produce totally synthetic glycosylated erythropoietin.17 These methods can be applied to the HIV/AIDS vaccine production. Attention has recently focused on the preparation, by chemical synthesis, of the V1V2 peptide-glycan epitope defined by BnAbs PG9 and CH01 lineages. In the context of these studies, a potential immunogen has been discovered that is capable of binding not only mature V1V2 BnAbs but also (in nM affinities) to their unmutated common ancestors (UCAs) (i.e., naïve B cell receptors (BCR)) of the V1V2 BnAb lineages. Previously, few Env constructs had been found that bind to the CH01 lineage UCA18 and none had been found that bind to the PG9 UCA.10 
The present invention results, at least in part, from studies involving the design and chemical synthesis of immunogens that elicit neutralizing antibodies directed toward the V3 glycan epitope that is recognized by mAb PGT128.